MAZATLAN BIVALVES 111 



inent by the hollowing out of the sides. Long. '025, lot. '035, 

 alt. '02. 



Hob. Mazatlan ; 1 valve off Spondylus ; L'pool Col. 

 Tablet 498 contains the specimen. 



158. LEPTON DIONJEUM, n. s. 



L. t. papyraced, alba, subtrigonali ; lavi, striulis incrementi 

 minimis confer tissimis : incequilaterali, marginibus dorsalibus 

 subrectis ; cardine dent. lat. duobus curtis, prominentibus, fos- 

 sam ligamenti kaud parvam continentibus. 



In the only valve found of this species, there is no cardinal 

 tooth. The shape nearly resembles some species of Dione, 

 with the dorsal margins nearly straight, and at right angles. 

 It is known at once from L. Clementinum by the absence of 

 cellular markings and concentric undulations, the place of the 

 latter being supplied by minute crowded striae of growth. 

 Long. '04, lat. '06, alt. '03. 

 Hab. Mazatlan ; 1 valve off Spondylus ; L'pool Col. 



Tablet 499 contains the specimen. 



159. LEPTON UMBONATUM, n. s.* 



L. t. subquadratd, subinflatd, cinered, opacd; striis incrementi, 

 item hue et illuc liris acutis concentricis ornatd ; umbonibus par- 

 vis, valde prominentibus ; margine ventrali compresso ; cardine 

 dent. lat. kaud longis, excurvatis ; fossa ligamenti valde depres- 

 sd ; impress, muse, subrotundatis. 



Shell Kellia-shaped, but with a small extremely prominent 

 umbo, and a flattened ventral edge. Surface of a dull ashy 

 colour, with an extremely thin epidermis ; rather solid, and 

 not displaying the shagreened pattern ; with the lateral teeth 

 short, but not so much so as in the other species, somewhat 



* As so little is known of this genus, I append a description of a perfect and 

 well marked valre at present in my collection. 



LEPTON PLACUJTOIDEUM, n. s. L. t. planata, quadrata, subincBquilaterali, 

 oentraliter incurva, papyracea, alba, hand punctata; liris concentricis acutis, ven- 

 traliter undnlatis, interstitiis striis exillimis concentricis; umbonibus parvis, sed 

 a Unea dorsali recta valde prominentibus ; fossa ligamenti parva ; dent. lat. parvis, 

 extantibus, divergentibus, in umbone sitis, admarginem dorsalem paululum excurva- 

 tis ; dent. card, altera in valva nullo, altera ? ; imp. muse, haud conspicuis. 



Hab. West Indies. Shaped like L. squamosum, but distinguished at once by 

 the prominent umbos, incurved ventral margin, sharp concentric ridges, and 

 absence of the shagreen pattern. Long. *07, lat. !, alt. -03. 



